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Human sperm have the ability to 'smell' and may even have a special receptor that allows them to sniff out an egg and make a beeline for it, a new study has found.

The identification of the receptor - and two compounds that either activate it or block it - may lead to new ways to promote or prevent conception, a team of German and US researchers report today in the journal Science.

A signalling system between egg and sperm "could be used to manipulate fertilisation, with important consequences for contraception and procreation," said the team's leader, Marc Spehr of Ruhr-Universitšt Bochum, in Germany.

Hundreds of olfactory, or odour, receptors have been discovered but, despite their name, they are often found in body tissues other than the sensory neurons of the nose and are coupled to complex biochemical signalling pathways.

The new receptor - called hOR17-4 - is one of several now known to be expressed mainly or exclusively on the surface of human sperm cells. When sperm were exposed to an odourant called bourgeonal, the researchers were surprised to find that it bound to the receptor and triggered a series of physiological events that caused the sperm cells to swim more strongly and towards the source of the bourgeonal.

"We were not expecting to uncover a receptor for chemo-attraction, this is the best we could expect to find," said Spehr, who noted that one of the next steps is to identify an equivalent compound to bourgeonal that may be produced by human females.

The scientists do not yet know if the egg itself or some part of the female reproductive tract produces such a substance. "Some of the difficulties experienced in IVF treatments may be linked to the quality of sperm. Bourgeonal might be used in the future to find the motile and fast sperm cells that are needed for fertilisation.

The researchers also identified an antagonist compound, called undecanal, that seems to inhibit the sperm response to the bourgeonal signal. "One could speculate about delivery of undecanal into the female genital tract or even about drugs containing equivalents to undecanal that could be used by men." said Spehr.

Sea urchin sperm are known to seek out sperm-attracting substances produced by free-floating sea urchin eggs, but scientists generally have been sceptical that such signals were involved in human conception. A question remains open, however, as to whether human eggs send out signals to select only some kinds of sperm - say, the strongest swimmers - from the wriggling army of millions released by a single male ejaculation. Spehr believes it is likely that other kinds of olfactory receptors will be discovered on the surface of human sperm. "I don't expect nature to be dependent on one receptor type," he said. "This is not the way I think nature works."